Virtual Power vs Stages

power-meter

#1

Hello, I just got a Stages left-side-only power meter after I’ve been using TR with virtual power for the past 5 months or so. I’ve noticed a lot of previous topics where people were describing virtual power as being something pretty large like 30 watts off compared to a power meter but overall pretty even. What I’ve noticed after one TR ride where I used virtual power on the computer (cycleops Fluid2 trainer) and the stages power on my garmin head unit was a pretty weird looking power band like this:

VP FTP 180
90w VP vs 40w Stages
170w VP vs 165w Stages
275w VP vs 310+w Stages

It seems like on really light spins, stages is showing nearly half the watts of virtual power. At or around my FTP (140w-200w or so) it seems consistent enough. It tends to be roughly ±10w. At well above, starting by say 120% of FTP and certainly by 150% the Stages is showing way more than virtual power.

Is this normal expected behavior of either a left-side only power meter or virtual power to have a weird curve like this?


#2

That’s the nature of VP. It is subject to equipment and setup variability. If your tire, tire pressure and roller pressure don’t match what TR did when they generated their VP curve, you will get variation. Add to that overall manufacturing tolerances of the trainer itself and you get wide variability.

VP is a great tool when you hold your setup consistent between workouts. But it’s not great for “accuracy” to a “real” power value.

As with any power measurement change, do a new Ramp test, use that data and ignore the past.


#3

Awesome thanks! I have an FTP test scheduled for the middle part of build on Tuesday anyway. I suppose this explains part of why VO2Max workouts seem extremely hard and recovery rides seem a little easy to the point where I was questioning their utility.


#4

The Stages is an internally consistent unit and as long as you aren’t introducing any other variables (other bikes with other power meters, etc), then it should work better than virtual power to track progress.